This invention relates to an apparatus for, and a method for, achieving temperature compensation for an avalanche photodiode used in a geodetic-distance meter or similar devices.
As well known, an avalanche photodiode, which is often used as a photosensitive element, has its multiplication factor varied according to the ambient temperature. In case an avalanche photodiode is used as a photosensitive element in, for example, a geodetic-distance meter, a bias voltage to be applied to the photodiode should be controlled according to the ambient temperature to prevent the multiplication factor from changing.
FIG. 1 is a block diagram of a known apparatus for achieving temperature compensation of an avalanche photodiode, thus showing also a known method therefor. As shown in FIG. 1, a voltage control circuit 1 is connected to a series circuit which is constituted by an avalanche photodiode PD and a load resistor R. The circuit 1 applies a predetermined bias voltage to the avalanche photodiode PD. The load resistor R is connected at one end to an amplifier 2, the output of which is connected to a signal filter 3. When the avalanche photodiode PD receives an input signal ray, it produces an output. The output of the photodiode PD is supplied to the amplifier 2 and the signal filter 3 and is taken out as a detection signal. A reference signal source 4 modulates an output of a light-emitting diode LED, using a reference signal having a frequency different from that of the detection signal, thereby obtaining a reference signal ray. The reference signal ray thus obtained is applied onto the avalanche photodiode PD. Upon receipt of the reference signal ray, the photodiode PD produces an output. This output of the photodiode PD is taken out through a reference signal filter 5, converted by a detector 6 into a DC voltage, and compared by a differential amplifier 7 with an output of a reference voltage source 8. The output of the differential amplifier 7 is supplied to the voltage control circuit 1, whereby the output voltage of the circuit 1, i.e. bias voltage of the photodiode PD is controlled.
If a negative feedback circuit is used to receive the reference signal ray, the output voltage of the detector 6 is equal to a reference voltage built up by the reference voltage source 8. In this case, the negative feedback circuit controls the bias voltage of the photodiode PD in such a manner as to maintain the multiplication factor of the photodiode PD unchanged, even if the ambient temperature changes.
The apparatus and method shown in FIG. 1, however, has some drawbacks. First, a temperature compensation should be conducted also on the light-emitting diode LED in order to achieve an accurate temperature compensation of the avalanche photodiode PD, for the output level of the light-emitting diode LED varies according to the ambient temperature, too. Second, the signal-to-noise (S/N) ratio of the detection signal often become poor since the reference signal ray is applied onto the avalanche photodiode PD while the photodiode is receiving the input signal ray. This is because, the more light it receives, the more noise an avalanche photodiode produces. Such deterioration of the S/N ratio of the detection signal is unavoidable in the apparatus of FIG. 1. The reference signal ray must be far more intense than the input signal ray because an input signal ray of, for example, a geodetic-distance meter is very feeble and because the gain of the negative feedback circuit cannot be made large but to a limited extent lest the negative feedback circuit should become unstable.